My little girl entered her estrous two weeks ago and contrary to my title, she was *very* energetic during the first two weeks. She ran everywhere and I think I might have lost a pound or two while trying to keep up. However on her 16th day's evening, she became quite lethargic. She did eat her food (her appetite was lower than usual during this period anyway which seems expected) and drink water, but she looks like she doesn't even have enough energy to walk. Today's the morning of her 17th day and I can't get her to go outside - we're 2 hours late than our usual time and she normally never says no to going for a walk. She seems happier than last night, she at least wags her tail when she sees me, but still...

Should I be concerned? I mean, I am already concerned, but is it something to worry about or something common? I read that lethargy is common during the heat cycle, but nobody mentioned a high energy period followed by a "I didn't get to mate" blues.

Sounds like a false pregnancy. The dogs body produces hormones, etc after the estrus period to ready the body for pregnancy. This can include swelling of the teet glands even though they wont produce milk. She might even be 'resting' so she saves up the energy to birth and nurse.